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Abstract

Background

It is widely held that in toothed whales, high frequency tonal sounds called 'whistles'
evolved in association with 'sociality' because in delphinids they are used in a social
context. Recently, whistles were hypothesized to be an evolutionary innovation of
social dolphins (the 'dolphin hypothesis'). However, both 'whistles' and 'sociality'
are broad concepts each representing a conglomerate of characters. Many non-delphinids,
whether solitary or social, produce tonal sounds that share most of the acoustic characteristics
of delphinid whistles. Furthermore, hypotheses of character correlation are best tested
in a phylogenetic context, which has hitherto not been done. Here we summarize data
from over 300 studies on cetacean tonal sounds and social structure and phylogenetically
test existing hypotheses on their co-evolution.

Results

Whistles are 'complex' tonal sounds of toothed whales that demark a more inclusive
clade than the social dolphins. Whistles are also used by some riverine species that
live in simple societies, and have been lost twice within the social delphinoids,
all observations that are inconsistent with the dolphin hypothesis as stated. However,
cetacean tonal sounds and sociality are intertwined: (1) increased tonal sound modulation
significantly correlates with group size and social structure; (2) changes in tonal
sound complexity are significantly concentrated on social branches. Also, duration
and minimum frequency correlate as do group size and mean minimum frequency.

Conclusion

Studying the evolutionary correlation of broad concepts, rather than that of their
component characters, is fraught with difficulty, while limits of available data restrict
the detail in which component character correlations can be analyzed in this case.
Our results support the hypothesis that sociality influences the evolution of tonal
sound complexity. The level of social and whistle complexity are correlated, suggesting
that complex tonal sounds play an important role in social communication. Minimum
frequency is higher in species with large groups, and correlates negatively with duration,
which may reflect the increased distances over which non-social species communicate.
Our findings are generally stable across a range of alternative phylogenies. Our study
points to key species where future studies would be particularly valuable for enriching
our understanding of the interplay of acoustic communication and sociality.

Background

Cetacean tonal signals are broadly defined as narrowband, frequency modulated sounds
[1-3]. Such sounds are produced by both baleen whales (Mysticeti) and toothed whales (Odontoceti)
– sister clades containing all extant whales. They are also produced by other mammals
[e.g., [4]] and thus appear primitively present in the order. Baleen whales produce sounds that
have fundamental frequencies generally below 5 kHz [2,5], as do members of the sister lineage of Cetacea, the hippos [4]. In toothed whales, in contrast, these sounds most commonly range from 5–20 kHz [2], and in some species, e.g. Delphinus delphis, Stenella attenuata, S. coeruleoalba, S. longirostris [6]Lagenorhynchus albirostris [7], Tursiops truncatus [8], fundamental frequencies can go as high as 48 kHz in Inia geoffrensis [9]. In delphinid toothed whales these high frequency tonal sounds, especially when complex,
are often referred to as 'whistles', although within the group whistle acoustic characteristics
vary enormously. Several species produce both frequency modulated whistles (e.g.,
sine, convex, concave, upsweep, downsweep) and simple whistles that are relatively
constant in frequency (e.g., Lagenorhynchus albirostris, [7]; Sotalia fluviatilis [10]; Stenella longirostris [11], others are limited to simple whistles (Lipotes vexillifer) [12] or to few frequency modulated whistles (e.g., mostly downsweep in Inia geoffrensis) [9]. In addition, whistle contour may be continuous or consist of a series of breaks
and segments [2]. Whistles may or not contain harmonics [2]. In delphinid species like S. longirostris [13] and L. albirostris [14] whistles can contain high order-harmonics. Finally, whistle duration is very variable.
For instance, in Sousa chinensis whistles can range from 0.01 to 1.3 seconds [15] and in Tursiops truncatus from 0.05 to 3.2 seconds [16]. In delphinids, whistle frequency modulation and duration varies within species in
relation to geography [e.g., [10,11,16,17]], and related species differ in many whistle frequency components (e.g., maximum,
minimum, end, and start frequency) [e.g., [18-22]].

Baleen whales produce a great variety of sounds, among them tonal sounds that like
toothed whale 'whistles', are narrowband and frequency modulated, although typically
much lower in frequency [1]. These tonal sounds can be produced in isolation or in combination with other sounds
(e.g. pulsative sounds). In the Right whale (Balaena glacialis) these tonal sounds, again like 'whistles' in toothed whales, are used in a social
context [23]. For example, in Blue whales (Balaenoptera musculus) tonal sounds are presumably used for long-distance communication [24], and in Right whales tonal sounds are used in combination with pulsative sounds in
a sexual context [25]. However, in baleen whales, these tonal sounds are never referred to as whistles,
but as 'calls', 'moans' or 'tones' [26,24-29]. Nomenclature of sounds, both in toothed and baleen whales, is confusing. As stated
by Au (2000: 31) [1] in baleen whales "as with dolphins there is a lack of any standard nomenclature for
describing emitted sounds", this frustrates comparison of sounds across taxa and obscures
homologies. It remains unclear exactly what is a 'whistle', and if narrowband, frequency
modulated tonal sounds of baleen whales and toothed whales are homologous at some
level. One reason to question tonal sound homology across whales is that the sound
production mechanisms of baleen whales and toothed whales are dramatically different.
In baleen whales tonal sounds are thought to be laryngeal [30,31], as they are in other related mammals [e.g. [4,32]], but in toothed whales sounds are produced by a unique and complex nasal system
[e.g. [33,34]]. This offers some support for the hypothesis that toothed whales 'whistles' are
unique and different from (not homologous with) baleen whale tonal sounds. However,
this also suggests that high frequency tonal sounds are homologous across toothed
whales and such sounds in non-delphinid toothed whales should also be called whistles
(contra Podos et al. 2002) [35]. To accommodate both possibilities we do all analyses across all whales (allowing
for potential homology of tonal sounds across the order) and separately within toothed
whales.

Most of the work on whistles has been done with social delphinids, where they are
often referred to as "social signals" and are thought to facilitate individual recognition,
group cohesion, recruitment during feeding activities, and overall communication [e.g.,
[1,3,36-44]]. Generalizations about the function of whistles have translated into the hypothesis
that whistles evolved in concert with sociality, and that the two traits are tightly
correlated [e.g., [45,35]]. Herman and Tavolga (1980) [45] suggested that the degree of gregariousness in toothed whales seemed to be related
to whistle production [see also [46]]. More specifically, they proposed that species that live in small groups or are
solitary tend not to whistle, whereas species that live in large groups frequently
do. Recently, Podos et al. (2002) [35] proposed that whistles are an innovation of social delphinids; in other words that
whistles are synapomorphic for a clade within Delphinidae. However, even within delphinids
some social species such as Cephalorhynchus spp and some species of Lagenorhynchus do not whistle [e.g., [46,47]], which seems to contradict the dolphin hypothesis. The hypothesis was furthermore
based on an assumption of the absence of whistles in river dolphins (Inia, Lipotes, Platanista, and Pontoporia), porpoises (Phocoenidae), beaked whales (ziphids) and belugas and narwhals (Monodontidae).
However, we do not believe this assumption is justified. Tonal sounds from Inia geoffrensis, for example, have been independently recorded in several studies [9,21,22,48]. These sounds, just like in other toothed whales, have been referred to as whistles,
although they are simpler and shorter in duration, and higher in frequency than the
whistles of some dolphins [9]. Similar whistles have also been reported in another river dolphin Lipotes vexillifer [e.g., [12,49,50]] and in social non-delphinid toothed whales such as some beaked whales [51,52], and the Monodontidae, belugas and narwhals [e.g. [53-57]]. Podos et al. (2002) [35] concluded that the tonal sounds in these species should not be classified as 'whistles',
and hence found support for the dolphin hypothesis. While we agree with Podos et al. that whistle structure seems different in delphinids and non-delphinid toothed whales
we believe this demonstrates the basic problem of treating broad, arbitrary, concepts
as single traits in evolutionary analyses. To define whistles as social sounds produced
by delphinids – a priori denying homology with tonal sounds in related taxa – and
then concluding that they evolved in association with sociality in Delphinidae risks
circularity. In such a framework reconstructing the origin of 'whistles' on a phylogeny
will simply depend on the whistle definition chosen by any given author.

To facilitate discussion, and comparability with previous research, we use the word
'whistle' for toothed whales tonal sounds, however, we do not imply that whistles
are necessarily non-homologous to baleen whale tonal sounds – their homology requires
further study. We use whistles as a category for some of our analyses, mainly to test
the dolphin hypothesis as it was proposed. It is not very informative, however, to
simply map the distribution of 'whistles' on a phylogeny (Fig. 1, [see Additional file 1]). Authors differ in their interpretation on the presence or absence of whistles
across species, e.g. some define them in the context of a behavior that may have much
more limited distribution than the sounds themselves. Furthermore, even within dolphins
'whistles' can be highly variable. We thus highlight the need to focus on the various
acoustic parameters (such as frequency variables, modulation, etc.) that may vary
independently and have non-identical phylogenetic distributions [see Additional file
1 for rationale]. Hence, our major focus is on such analyses which may reveal which,
if any, of the characteristics of 'whistles', or tonal sounds in general, seem associated
with sociality.

Additional file 1. Whistles as a unit for evolutionary analyses. As noted above there are several reasons
why using conglomerate concepts like 'whistles' as units of study can hinder progress
in the understanding of sound evolution. Apart from being rather arbitrarily defined,
and hence differently by different authors, 'whistles' represent a set of characters
that may vary independently and may each have different phylogenetic distributions.
As a thought experiment let us think of an example where sound production is being
compared in two sister lineages. Let us assume that some authors are interested in
the evolutionary origin of tonal sounds called 'snorts', and that snorts are defined
as narrowband, frequency modulated sounds, with a contour containing at least two
inflection points and frequency above 10 kHz. In group A it is noted that sounds are
narrowband, frequency modulated, with three inflection points and frequency ranging
from 12–15 kHz. In group B sounds are narrowband, frequency modulated, with a contour
of two inflection points and frequency ranging from 7–9 kHz. Under a 'broad concept'
analysis we would therefore conclude that 'snorts' were present in A, but absent in
B, and might conclude that snorts originated in the common ancestor of A (diagram
a). However, this belies both the similarities and differences that exist in sound
production in the two groups. It denies homology of frequency modulation, contours
etc, and even suggests that tonal sounds evolved independently in each group (as 'snorts'
are 'different' tonal sounds from non-snorts). Under a 'component' analysis (diagrams
b and c), traits like frequency modulation and band width would be scored as identical
in the two groups – their similarity would be taken as evidence of common ancestry,
i.e. homology. Instead of 'snorts' originating in A, we would more simply explain
the differences between the two groups in terms of frequency, and if e.g., the outgroups
shared the lower frequency (indicated by white branches) of B we would conclude that
a switch to higher frequency (indicated by black branches) occurred in the common
ancestor of A (diagram b). In other words, we would learn that the difference between
what people call 'snorts' and what they don't call snorts may simply be a matter of
sound frequency. In this latter case there is no indication of tonal sound production
being non-homologous in A and B, and in fact they share most characteristics of the
tonal sounds. Additionally we would learn (diagram c) that inflection points increased
from two (white branches) to three (dark branches) in the lineage leading to B (supposing
the condition in A was shared with the outgroups). This is information that the concept
of 'snorts' obscured. By a component analysis we learn a lot more than by a concept
analysis. If we now were interested in the association of sounds and sociality, and
group A was social and group B (and outgroups) not, it might be claimed that 'snorts'
and 'sociality' are associated and evolved in concert (following diagram a). However,
a much more precise and informative conclusion would be that sociality and sound frequency
(diagram b) might be related. Hence instead of explaining the social context of 'snorts'
we would do well to examine how sound frequency might play an important role in social
communication etc. We believe that 'whistles' are no better justified as a unit for
evolutionary analysis than 'snorts' in the example above. We do use them in an attempt
to test the dolphin hypothesis, but then we opt for a component approach for most
of our analyses.

Figure 1. Optimizations of tonal sounds (a) and whistles (b) versus sociality using the broad
concept approach [see additional file 1]. A brief glance at the black branches (indicating presence of tonal sounds/whistles
and 'complex' sociality) on each side does not suggest detailed correspondence of
acoustic structure with sociality. In other words whistles have a different phylogenetic
distribution than does complex sociality etc, indicating that their co-evolutionary
history (if any) may be more complicated than previously thought.

Our understanding of tonal sound acoustic structure, diversity, and use, is growing,
but the evolution of tonal sounds and their association with sociality remains highly
speculative. We therefore believe we here improve upon previous studies by providing
a more detailed analysis, and using novel and more detailed phylogenies than any study
hitherto. We also test these hypotheses across a range of alternative phylogenies.

In sum, we here review current knowledge of both tonal sound production and social
structure in Cetacea, and explore the evolution of tonal sounds and the association
of individual tonal sound components with sociality (overall social structure and
social components). Taking advantage of a new species-level cetacean phylogeny [58,59] we provide the first phylogenetic test of the hypotheses of Herman and Tavolga (1980)
[45] and Podos et al. (2002) [35]. This study identifies large gaps in knowledge on both traits, and points to key
species where future studies would be particularly valuable for enhancing our understanding
of the interplay of tonal sounds and sociality.

Results

Testing the Dolphin Hypothesis

The following is presented merely to test the dolphin hypothesis as stated (see Introduction,
Methods, and [see Additional file 1] for problems with this coarse approach). Under the definition of 'whistle' we use
here, the optimization of whistles on the phylogeny is ambiguous (Fig. 1b). However, all of the equally most parsimonious reconstructions reject the dolphin
hypothesis. The phylogeny implies that whistles either evolved independently twice,
once in Berardius and once in the node leading to Delphinida sensu Muizon (1988) [60], delphinoids plus river dolphins + Platanista (a clade we here refer to as Pandelphinida), with secondary losses in Phocoenidae
and within Delphinidae (Cephalorhynchus spp. and Lissodelphis spp.). Alternatively whistles evolved once in the common ancestor of ziphiids plus
pandelphinids and then were subsequently lost thrice in Hyperoodon, phocenids and within delphinids (the optimization of whistles is equally ambiguous
on previously published phylogenies, [see Additional files 2, 3, 4], while dual origin of whistles is better supported when optimized across the entire
set of filtered post-burnin trees, see Methods). Likewise, there are two possible
optimizations of sociality under a broad concept approach. One is that sociality evolved
in the common ancestor of Odontoceti and was then lost secondarily twice in the riverine
species (Fig. 1b). Alternatively sociality may have evolved independently four times (in Physeter macrocephalus, within Ziphiidae, Pontoporia, and in Delphinoidea). The optimization of sociality is ambiguous on over 99% of
the alternative trees examined, however, the multiple loss of sociality within Cetacea
seems more likely in general, given that relatives of whales are social. Regardless
of choice of optimizations, whistles did not originate in the lineage leading to the
social dolphins, contra the dolphin hypothesis.

Additional file 2. A cetacean phylogeny consistent with Arnason (2004). A majority rule consensus of
all post-burnin trees from May-Collado et al. (2007) filtered to be congruent with
the mitogenomic phylogeny of Arnason (2004). Numbers on nodes represent posterior
probabilities.

Character Optimizations

Results of character optimizations led to the same conclusions across all alternative
phylogenies examined (previously published hypotheses, [see additional files 2, 3, 4], and post-burnin trees from our Bayesian analysis of Cytochrome b), unless otherwise
noted.

Group sizes in Cetacea [see Additional file 5] appear to have been ancestrally small, but to have gradually increased in the lineage
leading to the dolphins, with a number of independent derivations of societies with
hundreds of individuals and some secondary reductions in group size (e.g., Cephalorhynchus spp, Orcaella and Orcinus Fig. 2).

Additional file 5. Cetacean social structure and group size. This table reviews published data on cetacean
social structure and group size. Numbers in parenthesis correspond to state assigned
to each characters as described in Table 1 (bold numbers represent the most common state reported for a particular species).

Figure 2. Optimization of group size in Cetacea (using natural log). Dark purple and blue colored
branches indicate small groups and demark most of the 'basal' whales. More brightly
colored (green, yellow and red) indicate larger groups. The phylogeny suggests gradual
increase in group size in the lineage leading to Delphinidae, with independent evolution
of huge groups (red) in several lineages and some reversals to smaller groups (e.g.
Cephalorynchus hectori).

Here we present some alternative optimizations of sociality under both a 'broad two
and four state concept' framework simply to test the dolphin hypothesis and under
a multiple component framework. We note, however, that our study offers limited insights
into the evolution of sociality in cetaceans. Future studies will require examining
a greater number of component characters of sociality as such data becomes available,
and it will require the inclusion of comparative social data also from the outgroups.

We compare three optimizations of sociality represented as a four-state character
(social structure) (see Table 1 and [see Additional file 5]). First, we keep polymorphic species (species reported to show more than one type
of social organizations) as such and then compare results when the 'lowest' and 'highest'
social state is chosen for each polymorphic species (Fig. 3). All three optimizations have some ambiguity, but optimizations across all trees
suggest that family based groups evolved independently at least three times (Physeter, Monodon, and Globicephalinae Fig. 3). The optimization of social components (including polymorphism) is shown in additional
material [see Additional file 6]. Group composition appears to have ancestrally been simple groups consisting only
of mother and calf. Segregated (by sex and/or age) and mixed groups may have evolved
independently at least four times [see Additional file 6]. Finally, member associations appear to have evolved from simple mother and calf
interactions to complex family based associations [see Additional file 6].

Additional file 6. Optimization of components of sociality. This figure shows social components optimization
(a = group size, b = group composition, c = group stability/association patterns)
on the preferred phylogeny. Note that this optimization contains polymorphic species
and thus family based group like Physeter and Monodon and species with long-term associations between non-related group members are all
optimized using the lowest state of sociality.

Figure 3. Optimizations of social structure as a four state character (a) leaving polymorphic
species as such, (b) lowest social state, (c) highest social state. All analyses were
done using the highest social state optimizations (see Methods).

Table 1. Definitions of sociality and tonal sound characters and respective states

Figure 4 shows the optimization of each acoustic character (all transformed using the natural
log). Relatively high maximum and minimum frequencies (both absolute and mean) appear
derived in toothed whales (Fig. 4a–b, d–e). Particularly high mean maximum and minimum frequencies have evolved within delphinids
(note that some of the variation within delphinids and other groups is visually masked
by the way Mesquite groups continuous variables in color ranges; [see Additional file
7 for greater detail].

There appears to be a similar trend in the number of tonal sound inflection points
(an indicator of tonal sound complexity) going from few ancestrally and increasing
in the lineage leading to the dolphins (Fig. 4f). There is an inverse trend in tonal sound duration, where particularly short tonal
sounds appear to be derived within the delphinids (Fig. 4c).

Character regressions and correlations

Under the independent contrast method the regression between group size and the mean
number of inflection points was marginally significant: species with larger groups
tend to produce tonal sounds with greater mean number of inflection points. Group
size explained approximately 7.9% of the variation in inflection points across cetaceans
(p = 0.05, df = 33 see Fig. 5 (this and some of the following results are dependent on the choice of phylogeny,
see section Phylogenetic uncertainty). Group size also significantly explained variation in the mean minimum tonal sound
frequency within toothed whales (R2 = 12.4%, df = 23, p-1tailed = 0.04). We justify using a one-tailed test based on the expectancy that low frequency
sounds travel longer distances so that a priori one might expect that low frequency
tended to be associated with solitary species, while species that live their entire
lives in large groups need only communicate over short distances. However, given that
the two tail test is non-significant we consider this hypothesis only weakly supported.
Regressions between group size and other acoustic parameters were not significant.

Figure 5. Regression analysis between independent contrasts of mean group size and mean number
of inflection points. One conspicuous outlier (arrow) represents a contrast including
the killer whale (Orcinus orca) which forms relatively small social groups but produces highly modulated whistles.
It has been proposed that the killer whale uses whistles in a manner different from
any other delphinid to indicate motivational state. That multiple factors are at work
shaping tonal sounds in cetaceans may obscure and make difficult to discover true
co-evolutinary histories of characters. Accordingly when O. orca is removed from the analysis the regression between the two characters becomes stronger.

Changes in tonal sound complexity were significantly concentrated within social lineages
in four of the five most parsimonious reconstructions when both traits were treated
as two state characters [see Additional file 8] .

Tests of character state associations (SIMMAP) show that complex whistles (state 1
= more than one inflection points) were positively associated with group living species
(Dij = 0.13, p > 0.999) and negatively with less social species (Dij = -0.024, p <
0.001) treating social complexity as a two state character. In general there was an
association between tonal sound complexity and social structure (Dstatistic = 0.376, p < 0.001, Table 2). However, the associations between individual states vary depending on how finely
tonal sound and social characters are divided (Table 2). For instance, when treating social complexity as a four state character but tonal
complexity as a two state character we find a significant positive association between
highly social species (states 2 and 3) and complex tonal sounds and a negative association
between complex tonal sounds and 'solitary' (state 0) species (Table 2). When both are treated as four state characters only negative associations are significant
(but in the same directions as before, see Table 2).

Table 2. Probabilities of association between sociality (selecting the highest social state
for polymorphic species) and tonal sound complexity. Significant positive associations
at p-values > 0.972 and 0.973** for two and four state complexity characters, respectively
and significant negative associations at p-values < 0.028 and 0.027* for two and four
state complexity characters, respectively

Additional file 9. Association between components of sociality and tonal sound complexity. This table
summarizes results from SIMMAP analyses of character associations between social components
(selecting the highest social state for polymorphic species) and components of tonal
sound complexity on the preferred phylogeny.

Phylogenetic uncertainty

In general, most of our findings are not strongly dependent on the phylogeny of choice,
as long as all the species are included. In other words, results in most cases are
similar whether the data are analyzed across the trees favored by our own analyses
(all post burnin trees and post burnin trees filtered using agreement among multiple
studies), or restricted to trees filtered to be congruent with the alternative hypotheses
of Messenger and McGuire (1998) [61], Nikaido et al. (2001) [62] or Arnason et al. (2004) [63], respectively (see Methods for detail). On the all-species phylogenies results significant
in the main analyses were also significant across all sets of trees for all SIMMAP
analyses. The only difference between analyses was that social and whistle character
states were more strongly associated on the trees constrained by the Messenger and
McGuire hypothesis than in the remainder [see Additional file 10]. Similarly, the PDAP analyses results agree irrespective of phylogeny choice [see
Additional files 11 and 12], except the following. Group size and number of inflexion points correlate significantly
except on trees constrained by the hypotheses of Arnason et al. (2004) [63] or Nikaido et al. (2001) [62], and group size and mean minimum frequency correlate except on trees constrained
by the Messenger and McGuire (1998) [61] hypothesis. For ancestral character reconstruction under parsimony, the optimizations
of the continuous characters such as group size, tonal sound frequencies, duration,
and inflexion points are nearly identical across the trees considered. The optimization
of whistles as a presence/absence character was ambiguous on our, and previous, phylogenetic
hypotheses. However, on 70% of the filtered post-burnin trees dual origin of whistles
was preferred (see above). The optimization of sociality (as a two state character)
was ambiguous (single origin followed by multiple losses, or two origins followed
by fewer losses), except on the Nikaido et al. (2001) [62] hypothesis which favors two origins of sociality. Similarly optimizations of whistles
and sociality as multistate characters varied little across trees with no impact on
conclusions.

Additional file 10. Association between sociality and tonal sound complexity. This table summarizes results
from SIMMAP analyses of character associations between social structure (categorized
as 1–4) and tonal sound complexity on the preferred phylogeny across reference phylogenies
(see Methods).

When we used the phylogenies resulting from reanalyzes of the data of Messenger and
McGuire (1998) [61], however, significance was lost in a higher number (although not the majority) of
the hypotheses tests [see Additional files 10, 11, 12] and some character optimizations changed. Although this can in theory imply sensitivity
to phylogenetic pattern, a simpler explanation for this finding seems to be that much
of the power of the comparative tests is lost as Messenger and McGuire's data [61] includes only a portion of the species of our main dataset. Hence we do not see a
reason to discuss these 'disagreements' further.

Discussion and conclusion

Our results show that the interplay of tonal sounds and sociality is complicated and
that studying the relationship between conglomerate characters such as 'whistles'
and 'sociality' largely conceals these intricacies. Under the very simple 'concept
approach' the cladistic test [see [64]] rejects the dolphin hypothesis stating that 'whistles' evolved as an adaptation
for social communication in dolphins. Whistles, as here defined, appear to be a synapomorphy
of pandelphinids, or even a more inclusive group including ziphiids (Fig. 1b). Therefore, the current evidence implies that whistles arose earlier in the evolutionary
history of whales than presumed by Podos et al. (2002) [35], and whistles are furthermore present in some non-social species, and have been lost
more than once within social clades. Apparently then, whistles are not necessary for
functional cetacean societies and social communication, and they can play some role
in communication in solitary species.

Our findings highlight some of the problems with evolutionary analyses of imprecise,
broad concepts. Even though 'whistles' do not correlate with any measure of sociality
we find evidence that the evolutionary histories of sociality and tonal sounds are
intertwined in the direction suggested by many authors, including Podos et al. (2002) [35]. This is evidenced mainly by two findings. (1) The significant association between
group size and tonal sound inflection points (complexity) whether tested using independent
contrasts, concentrated changes, or character association tests; and (2) the association
between group size and minimum tonal sound frequency (and the association of the latter
with duration). Simple tonal sounds are mostly confined to species with simple societies
(mostly solitary) such as river dolphins and rorquals while tonal sound and social
complexity increase in the lineage leading to Delphinoidea (Tables 2). Within that lineage reversal to simpler societies has occurred twice and each time
tonal sounds have been secondarily lost (Figs 1a–b, 3), although whistle loss may represent a response to predatory pressure rather than
change in social structure (see below).

In addition, especially in toothed whales, species emitting longer tonal sounds tend
to show a greater number of inflection points. These observations and tests are congruent
with hypotheses stating that complex tonal sounds function as social signals for group
cohesion (e.g., most delphinids) during social, traveling, and feeding activities
[e.g., [42,65]] or individual recognition (e.g., bottlenose dolphins, Atlantic spotted dolphins)
[e.g., [3,37,41,66,67]].

But functionality in a social context can only explain a portion of the variation
in tonal sound production and complexity. The secondary loss of tonal sounds in porpoises
and the dolphin clade containing Lagenorhynchus australis, L. cruciger and Cephalorhynchus spp, for example, suggests these signals may sometimes be costly, for example in terms
of energy production or predation risk. These odontocetes live in very fluid societies
where acoustic communication is accomplished by means of rapid pulsed sounds [47,68]. One potential costs of tonal sounds is that these signals may be intercepted (eavesdrop)
by an unintended receiver [69,70]. Delphinid tonal sounds are within a frequency range that is readily detected by
predators like killer whales which are known to predate on many marine mammal species
including these non-whistling species. Furthermore, porpoises and Cephalorhynchus seem to have converged upon similar morphology and biosonar systems [71,72], both have ears tuned for high frequency sounds and produce narrowband clicks [73] that are used for echolocation purposes and communication [74,75]. As emphasized by Morisaka and Connor (2007) [76] if killer whales poorly detect these signals, then it may be beneficial for these
species to use high frequency signals for social communication [73,74] instead of tonal sounds.

In stable societies like those of Physeter macrocephalus and Orcinus orca, animals tend to produce group-specific sounds (termed codas and calls respectively)
whereas in fission-fusion societieslike those of Tursiops truncatus and Stenella frontalis, animals produce individual-specific whistles, so called "signature whistles" [see
[3,15,41,38]]. Signature whistles are sounds (single-loop and multiple-loop) [see [75]] that to date have only been found in species with fluid societies where mother and
calf use them as contact calls and some animals (particularly males) form coalitions
(individual recognition may be important when forming these alliances) [e.g., [15,37,38,44,66,67,73,77-82]].

We found evidence for association between group size and the mean minimum frequency,
as well as between mean minimum frequency and duration. Given that the former was
only marginally significant, we will not place much emphasis on this finding. However,
if this finding will be better supported with the addition of further data it may
suggest that low minimum frequency (and long duration) is selected for in mostly solitary
species which must communicate with other individuals over relatively greater distances
than do species that live in permanent societies. It should be noted that May-Collado
et al. (2007) [59] found a correlation between minimum frequency and body size across whales. This may
explain a part of the observed pattern here, as social species are often small, but
it remains to be explored if sociality and body size are correlated.

Despite the possible differences in the context in which tonal sounds are produced
by riverine dolphins and other delphinoids, there is no a priori reason to assume that whistles produced by these toothed whales are not homologous
(contra Podos et al. 2002) [35], and phylogenetically their homology is supported (Fig. 1). It has been proposed that marked deviations of Inia from delphinids in scaling relationship in body size and frequency [e.g., [21,83]] is evidence that their sounds are produced by mechanisms different from those used
by delphinoids. This is primarily based on the assumption that vertebrate scaling
of vocal frequency occurs through size-dependent effects on a common vocal apparatus
[e.g. [80]], thus deviations from scaling relationships might indicate an independent proximate
mechanism [35]. However, these scaling patterns, for maximum frequency disappear once phylogenetic
relationships are taken into account [59].

While some cetacean societies have been studied for a long time, detailed observations
are lacking for many species and it is difficult to define and compare levels of sociality
across cetacean species. Likewise there are many gaps in our knowledge of tonal sound
production [see Additional files 5 and 7]. Our study highlights critical gaps in knowledge, and pinpoints key taxa whose future
study could quickly enhance our understanding of the evolution of tonal sounds. As
can be seen in Figure 1, tonal sound data would be especially valuable from Kogia, ziphiids other than Berardius, and from Platanista and Pontoporia. In a similar manner information on social structure of Kogia, Mesoplodon, and Ziphius would help resolve the optimization of sociality.

Many factors in addition to sociality have been proposed to have influenced the evolution
of tonal sounds, including body size and maximum frequency scaling [21,35,59,83,84], habitat [21], predation [76], and zoogeographical [20] and phylogenetic relationships [20,21]. Given that multiple factors are at work true co-evolutionary histories of any given
characters could easily be masked. Hence, finding significant correlations between
tonal sounds and social structure is particularly interesting. For example, we find
a significant, but rather weak, correlation between group size and inflexion points
using the independent contrast method. One of the conspicuous outliers in this analysis
is Orcinus orca, a social delphinid living in relatively small groups that nevertheless produces
extremely modulated whistles. Thomsen et al. (2001) [85] discuss these extreme modulations and suggest that whistles in killer whales serve
a different function than in related dolphins. Removing O. orca from the analyses increases the strength of the correlation between whistle complexity
and group size (R-square = 9.7%, p-value = 0.03). It should furthermore be noted that
comparative biology is fraught with difficulty, getting enough data together for a
strong hypothesis testing is typically difficult and missing data results in a loss
of power. By accounting for uncertainty in phylogenetic relationships we hope to reduce
the rate of type I error. Further, accounting for differences in interpreting and
scoring whistle and sociality data attempts to reduce type I error. It is quite possible
that in an attempt to avoid type I error we are introducing an unacceptable amount
of type II errors. In other words, our ability to detect true character correlations
in evolutionary history may be compromised. In this study, however, most of the results
were not sensitive to choice of phylogeny or alternative scoring scenarios which adds
some confidence to our conclusions.

Our findings point to gaps in knowledge of both tonal sounds and social structure
that need to be filled to significantly advance our understanding of their putative
co-evolutionary histories. Nevertheless, our results allow us to reject the simple
hypothesis that 'whistles' evolved for social communication in dolphins. However,
group size explains some of the variation in tonal sound frequency and frequency modulation
indicating a special role for complex tonal sounds in a (complex) social context and
perhaps for low frequency, long-duration sounds in solitary species. May-Collado and
Wartzok (2007) [9] suggested that whistles in Inia geoffrensis may be use to keep distance between animals rather than to stimulate social interactions.
However, this hypothesis needs to be tested. Future studies should focus on particularly
poorly known groups of species such as riverine species, ziphiids, and Kogia spp.

Methods

Definitions

For purposes of this study the association between tonal sounds and sociality will
be studied under both a broad concept [tonal sounds and whistles versus sociality,
emulating previous studies], and using a 'component' approach whereby tonal sounds
and sociality are dissected into (some of) their component characters. For tonal sounds,
standard acoustic parameters we use here include absolute and mean minimum and maximum
frequencies (kHz), duration (s), and number of inflection points (a measure of whistle
modulation, and a proxy for whistle complexity) [see Additional file 7].

Current knowledge on cetacean sociality indicates the existence of a wide range of
social structures, ranging from 'solitary' to highly structured group living species
[see [86]]. Generally in the study of cetacean sociality, social species are those that show
evidence of group living [87] where animals are associated in a nonrandom fashion [88]. Under the broad concept approach, we have classified species into two general social
frameworks, one simply organizing species into non-group living species (state 0)
and group living species (state 1) and a second one assigning species to four social
types (Table 1, [see Additional file 5]. Under the component approach, we also examine some component characters of sociality
for which there is sufficient data available (group size, composition, and stability/associations)
either from short and/or long term studies as well as anecdotal observations (Table
1, [see Additional file 5]). Table 1 provides detailed descriptions of these character and their states. It is important
to note that for any type of qualitative characterization of sociality, some species
may fit into more than one category due to intraspecific variation. For instance,
some populations of Stenella longirostris have unstable (or 'fluid') groups whose compositions change throughout the day, while
populations in the Hawaiian atolls exhibit long-term group fidelity and social stability
[89]. These, and other limitations of this study should be kept in mind when interpreting
our findings, nevertheless, we believe our approach improves upon previous attempts
to detect the associations between sociality and tonal sound production in whales.

Character Optimizations

Published data on cetacean tonal sound production and sociality were obtained from
literature and personal communications [see Additional files 5 and 7]. For tonal sounds we compiled information on the most used acoustic parameters:
absolute and mean minimum frequency, absolute and mean maximum frequency, duration,
and mean number of inflection points. We only considered studies conducted in the
wild or in captivity where, based on the information provided by the authors, it could
be assumed species were not recorded in mixed-species groups. We assumed authors were
not including harmonics in the acoustic measurements of the tonal sounds emitted by
the studied species, unless specified. Information about the social structure of cetaceans
was obtained from short to long-term studies, as well as anecdotal information. We
searched for information for each of the following social components group size, composition,
stability and associations patterns. In addition, information on these social components
was used to define four social categories. A minimum of two components was required
to place a species within a social category as defined in Table 1. Species for which insufficient components were available were coded as unknown.
For species with populations that varied in their social structure or any of the social
components ('polymorphic') we selected the highest social state for that particular
character. Group size is analyzed as a continuous character using the highest mean
group size found in the literature, and also as a discrete character which allows
the inclusion of more species [see Additional file 6] since many authors do not provide a mean value but instead offer a description of
group sizes.

We relied upon the recent species level phylogenies provide by May-Collado and Agnarsson
(2006) [58] and May-Collado et al. (2007) [59]. All the main analyses were made using the preferred tree from May-Collado et al. (2007) [59] [see Additional file 13]. Because polytomies can compromise character optimization and tests of character
correlations, characters were optimized on a fully resolved tree, which is the majority
rule tree resulting from a MrBayes analysis (see May-Collado and Agnarsson 2007 for
details) [58] without collapsing nodes with less than 50% frequency (using the contype = allcompat
option). However analyses were also run on a range of alternative phylogenies (see
below) Character optimization was performed with the program Mesquite 1.12 [90], using weighted squared-change parsimony [91].

Additional file 13. Phylogeny of Cetacea. This figure reproduces the preferred phylogenetic hypothesis
of May-Collado et al. (2007), used here for all main analyses. Numbers on nodes represent
posterior probabilities.

Acoustic characters were optimized in two data sets (1) with of all cetacean species
and (2) pruning species that are known not to emit tonal sounds, species for which
acoustic behavior is poorly known, and species that are known to produce tonal sounds
but for which detailed information for the character under study was not available.
When several values were reported in a species for a particular trait the largest
maximum frequency and duration, and the smallest for minimum frequency were used for
the analyses [see values in bold in Additional file 7]. Number of inflection points was analyzed both as continuous, reflecting the continuous
nature of the data, but also as a two and four state discrete character to facilitate
additional analyses that require ordinal data (Table 1, [see Additional files 5 and 9]).

Sociality was optimized as discrete two and four state characters, and using the social
components: group size, composition, stability and association patterns (Table 1, [see Additional file 5]). Because several species were polymorphic for one or several characters we optimized
species in three ways (1) as polymorphic, (2) emphasizing their 'highest' social level
reported, and (3) emphasizing their 'lowest' social level reported. Finally, we analyzed
group size as a continuous character.

Independent Contrasts

Assuming group size as a coarse proxy for social complexity (as defined above by Connor
2000) [87] we regressed it against tonal sound parameters to examine the association of sociality
and tonal sound production. Contrasts were calculated using the method of phylogenetically
independent contrasts [92]. The method takes into account known dependencies among observations due to phylogenetic
relationship of species, and therefore reduces error [93]. Independent contrasts were calculated using the PDAP: PDTREE module [[94], using an unpublished version provided by P. Midford] in Mesquite 1.12 (build h47,
85). To estimate independent contrasts, branch lengths were used as estimated by MrBayes;
branch length transformations were necessary for group size (Lack of fit test p <
0.05) and were exponentially transformed. We also tested the relationship between
tonal sound frequency and complexity [mean number of inflection points] and tonal
sound duration using the independent contrast method.

Character correlations

We also tested character associations between discrete characters of sociality and
tonal sound complexity using two different methods. First we used the software SIMMAP
1.0 [95] which allows for multistate character associations. We did the following tests using
all post-burnin trees (n = 2000) from our Bayesian analysis (May-Collado et al. 2007) [59] using default settings of the program and employing a rough false discovery rate
(FDR) to correct for multiple simultaneous comparisons (critical p values for tests
of 8, 12, and 16 comparisons are 0.028 (0.972), 0.027 (0.973), and 0.27 (0.973), respectively).
We tested the association of (1) sociality and tonal sound complexity both scored
as two state characters, (2) social structure and tonal complexity scored as four
state characters, and (3) each of the social components and tonal sound complexity
scored as two and four states characters [see Additional file 5]. Second using the concentrated changes test [96] in the software MacClade [97] we tested if changes in tonal sound complexity were concentrated on social branches.
For this test we used only two state characters.

It is important to note that testing the role (if any) of sociality in tonal sound
evolution is challenging due to the large gaps in our knowledge of cetacean societies,
difficulties of objectively defining tonal sound complexity, and levels of sociality,
and the limitations of available methods. We note that, as with all of the ordinal
data we use here, the divisions between character states are rather arbitrary and
open to criticism and alternative coding. Nevertheless we believe that our, be it
coarse, phylogenetic approach represents an advance over previous studies that have
speculated on social and whistle evolution using less data and lacking a phylogenetic
reference. We have tried to test the association of characteristics such as group
size and whistle parameters using various different approaches (independent contrast
test, concentrated changes test, pairwise comparisons on the phylogeny, and character
association test for multistate characters), testing them across various alternative
phylogenies, and our results are presented in the form of hypotheses that we hope
will subsequently be better tested upon the availability of more data and more sophisticated
methods. Also, importantly, our data highlight gaps in knowledge and should guide
future studies to where allocating resources might be most beneficial.

Current Knowledge on Cetacean Sociality and Tonal Sounds

Connor et al. 1998 [86] and Matthews et al. 1999 [83] provided brief reviews of the evolution of sociality in toothed whales and tonal
sounds in cetaceans, respectively. Connor et al. 1998 [86] review highlighted the lack of knowledge for most toothed whale species and focused
on the social structure of a few species including Tursiops truncatus, Orcinus orca, Globicephala spp., Berardius bairdii, Physeter macrocephalus. They compared toothed whale social structure with some terrestrial mammals e.g.
elephants and chimpanzees, and found both similarities between the two, but also identified
some social elements unique to toothed whales. Matthews et al. 1999 [83] summarized the frequency and time parameters of 40 cetacean species tonal sounds
in relation their body size.

This review summarizes information from 335 sources on sociality and tonal sounds
for 64 and 36 Cetacean species, respectively [see Additional files 5 and 7]. The information was gathered from via searches on Web of Science and Google Scholar,
and include scientific papers in peer-reviewed journals, conference abstracts, M.Sc.
theses, Ph.D. dissertations, technical reports to international organizations, etc.

Although not the main aim of this paper, a few summary statements can be made about
current knowledge of sociality and tonal sound production in whales [see Additional
files 5 and 7]. Baleen whales have a rather uniform social structure, generally live in simple
societies where animals spend considerable time solitary. Weak associations are limited
to aggregations form during the breeding and feeding time, and long-term associations
appear to be limited to the time mother and calf remained together. In contrast, toothed
whale social structure varies enormously, ranging from solitary to species living
in huge groups. In groups, group members show an array of association patterns, from
weak to stable family associations. For porpoises (Phocoenidae) and several of the
freshwater cetacean species (e.g., Platanista, Lipotes, Inia) authors have described group member associations as 'undeveloped', 'weak', or 'fluid'.
Such description are difficult to interpret and do not necessarily mean that the authors
are suggesting these species live in a fission-fusion society as reviewed in Connor
et al. 1998 [86] for Tursiops truncatus. For most delphinids, association patterns have been described as 'fluid', 'highly
fluid fussion-fusion', or 'fluid with short-lasting associations'. In these cases
authors appear to imply by 'fluid' that the species do live in fission-fusion societies
[as described by [86]]. In these species males tend to form coalitions and alliances to 'capture' and maintain
consortship with females. Finally, the most stable social structures have been described
in the Sperm whale, (Physeteroidea), most members of the subfamily Globicephalinae,
and possibly the Narwhal (Monodontidae). Notably, these species are not all closely
related so that "stable" societies have evolved convergently, however, species differ
in the degree of dispersal particularly male dispersal from the group.

Our review updates Matthews et al. (1999) [79] review on Cetacean tonal sounds. We included recently reported information on species
like Delphinus capensis and Sotalia guianensis [see Additional file 7]. We also updated information on several others like the Narwhal and Beluga (Monodontidae)
and the river dolphins Lipotes and Inia where more data has become available. The previous review [83] included tonal sound information from two beaked whale species (Mesoplodon densirostris, M. carlhubbsi) that we considered controversial due to the possible pulsative nature of these sounds,
thus exclude this information from the table. In addition, Sousa chinensis and Sousa plumbea were considered here a single species, since no clear evidence yet exists to separate
them into two distinct species. Likewise, we consider Stenella plagiodon as a synonym of Stenella frontalis.

Despite of the increasing knowledge on sociality and tonal sounds the information
remains lacking, or scattered, for many species. Here we are highlighting some of
these species, particularly key species in the phylogeny that would 'resolve' the
ambiguities observed in the evolution of sociality and tonal sounds.

Pygmy and Dwarf sperm whales (Kogia breviceps and K. sima) [98] are close relatives of the Sperm whale (Physeter macrocephalus) a species that shows a matrilineal society and does not produce tonal sounds. There
are no indications that these species show a similar society to that of the Sperm
whale. In general their social structure and acoustic signals are poorly known [99-104]. Pygmy and Dwarf sperm whales are often seen and strand in small groups that are
can be segregated by age and sex or mixed [102], [see Table 1]. The few published accounts on their sounds describe click trains [99,101,103] and cry-like sounds [104] but no tonal sounds.

Beaked Whales (Ziphiidae) are largely unknown. The social structure of the Northern
Bottlenose Whale (Hyperoodon ampullatus) is the best known of all beaked whales [e.g, [105-109]]. The Baird's Beak Whale (Berardius bairdii) is believed to live in stable groups where males may perform parental care [e.g.,
[86,110,111]]. However, other sources suggest these species live in fission-fusion societies [51]. However both sources report anecdotal evidence and long-term studies are necessary.
The social structure of other beaked whales is largely unknown. In terms of tonal
sounds, Winn et al. (1970) [112] reported whistles in H. ampullatus, but it appears to be the general consensus that this species does not produce tonal
sounds [e.g. [109], Whitehead pers. comn. 2005]. Tonal sounds have been reported as well in the Cuvier's
beaked whale, Ziphius cavirostris by Manghi et al. (1999) [113] but other acoustic studies only recorded pulsed sounds [e.g., [114,115]]. The only beaked whales for which tonal sounds have been reported are the Baird's
Beaked Whale [52] and the Arnoux's Beaked whale (Berardius arnuxii) [51]. There is some possibility that the recordings of Dawson et al. (1998) [52] were of a sympatric dolphin species (Dawson pers. comm.), however, the recordings
of Rogers and Brown (1999) [51] seem conclusive.

Inia, Platanista, Lipotes, Orcaella, Neophocaena live in freshwater environments. Generally riverine species are considered solitary,
however in some areas these species are often seen forming small groups [see Additional
file 1 and respective references]. Although, most authors describe group member interactions
in riverine species as weak, there is really little knowledge about their societies.
In terms of sound production, like the rest of the family (Phocoeenidae) [2], Neophocaena does not produce tonal sounds instead the species emits burst pulses under social
context [2]. Tonal sounds have been described for two of the subspecies of Inia geoffrensis, Lipotes vexillifer [see Additional file 7], but not for Pontoporia [116]. Mizue et al. (1971) [117] reported whistles from Platanista gangetica, recorded in captive conditions. However, it is not clear if the animals were acoustically
isolated from another riverine dolphin (I. geoffrensis), which produces tonal sounds.

The dolphins Lagenorhynchus cruciger, L. australis, Lissodelphis spp, Steno bredanensis, Feresa attenuata, and Peponocephala electra social structure is largely unknown. Most available information comes from stranding
and anecdotic information. Although Fish and Turl (1976) [118] documented whistles in Lissodelphis spp., recent work did not find whistles (Oswald pers. comn). No published accounts
on tonal sounds for Feresa and L. cruciger were found. May-Collado and Agnarsson (2006) [58] predict that L. cruciger may not emit whistles as it nests within a clade of species that do not.

Phylogenetic uncertainty

Taking phylogenetic relationships among species into account is crucial for hypotheses
testing in comparative biology. However, this is no simple procedure – phylogenies
themselves are merely hypotheses and for any given comparative study the number of
possible alternative phylogenetic arrangements grows exponentially with the number
of species being considered. The key question then becomes, how dependent are our
conclusions on the choice of phylogeny? Do the results remain mostly unchanged – implying
robustness to phylogenetic uncertainty – or do they change when tests are run on alternative
"reasonable" phylogenies. Alternative phylogenies can come from several sources, e.g.
from previously published independent phylogenetic studies, or from the set of near-optimal
trees from a given analysis, e.g. each unique tree from the post burnin set of a Bayesian
analysis. If the results of the comparative analyses are different under some of the
alternative phylogenies we have not rejected our conclusions but we have been cautioned
that the conclusions are dependent on the chosen phylogeny and may be altered as new
phylogenetic data become available. If, however, the results are the same across the
set of alternative phylogenies then confidence is gained in the conclusions. Here,
we attempt to account for phylogenetic uncertainty using various approaches.

The total number of trees in the post-burnin set from the Bayesian analysis is 2000.
Instead of basing sensitivity analyses on the 95% credibility set (which includes
a number of trees that contradict all recent studies of whale phylogenetics) we use
all the post burnin trees filtered based on various constraints reflecting external
phylogenetic evidence. This filtering reduces the number of trees facilitating analyses,
without much risk of compromising concerns for phylogenetic uncertainty as the constrained
clades are, by any standard, uncontroversial. Rather, considering trees that contradict
all available phylogenetic evidence would seem more likely to be misleading than useful.
Here, we (1) ran analyzes across the post-burnin set of trees from May-Collado et al. (2007) [59] filtered by constraining major clades all recent phylogenetic studies of Cetacea
agree have supported (see below), and (2) using subsets of the post-burnin trees filtered
so as to be congruent with other recently published phylogenetic hypotheses of cetaceans
chosen as they are based on various types of data: morphological/palaentological (Geisler
2003) [119], mitogenomic (Arnason et al. 2004) [53], a combination of molecular and morphological data (Messenger and McGuire 1998) [61] and SINE's (Nikaido et al. 2001) [62]. We chose to use previously published phylogenies as guides to filter trees from
the Bayesian post-burnin tree set, rather than to use them directly for analyses (but
see below). This is simply because each of these phylogenies contains only a small
subset of cetacean species making them poor for the purposes of comparative analyses.
Nevertheless, they represent relatively well supported and conflicting hypotheses
on the interrelationships of some of the major cetacean clades, whose resolution may
impact the findings of our study. Finally, we ran analyses on trees resulting from
re-analyses of the Messenger and McGuire dataset, which is the most taxon-rich previously
published phylogeny.

We constructed constraint trees in McClade [see Additional file 2] representing each of the previously published phylogeny (see above) and filtered
trees from the post-burnin set based on these constraint trees. The constraint trees
merely reflect the interrelationships of major clades (families and more inclusive
clades, [see Additional file 2]). Species level relationships are not constrained as most of the studies include
very few species so that they represent poor tests of lower level phylogenetic structure.
Finally, we produced one constraint tree representing only clades that all the previously
published studies agree on. This filtering process produced the following datasets:
Arnason constraint set (325 trees), Nikaido constraint set (341 trees), Messenger
and McGuire constraint set (4 trees), and the all study agreement constraint set (1069
trees). None of the post-burnin trees were congruent with the hypothesis of Geisler
(2003) [119]. In fact all other recent molecular, morphological, and combined analyses refute
aspects of that hypothesis, in particular the monophyly of all river dolphins (other
studies all agree that Platanista is not closely related to the remaining river dolphins),
and the monophyly of Physeteroidea (other studies refute the sister relationships
of Ziphiidae and Physeteridae). Hence we did not further consider that hypothesis,
although it played a role in the construction of the 'all study agreement' subset.

SIMMAP analyses were run across all trees in each subset, while PDAP analyses were
conducted on the majority rule tree (using contype = allcompat) of each of the subsets.
Furthermore, parsimony ancestral character reconstructions were examined on each of
the majority rule trees and across all trees from the all study agreement tree subset.

Authors' contributions

LJMC, DW, and IA designed the study. LJMC collected the data. IA and LJMC carried
out phylogenetic and statistical analyses, and drafted the manuscript. DW assisted
with multiple drafts of the manuscript. All authors have read and approved the final
version of the manuscript.

Acknowledgements

We thank Mike Heithaus, Volker Deecke, and three anonymous reviewers for comments
on this manuscript. We also thank Maureen A. Donnelly, Tim Collins, and Zhenim Chen
for their support, Tim Collins, Wayne P. Maddison and Peter Midford for advice on
some of the analyses and P. Midford for making available to us an unpublished version
of PDAP. We thank Julie Oswald and Shannon Rankin for providing us with unpublished
acoustic data. Funding for this project came from Judith Parker Travel Grant, Lerner-Gray
Fund for Marine Research of the American Museum of Natural History, Cetacean International
Society, Latin American Student Field Research Award of the American Society of Mammalogists,
the Russell E. Train Education Program-WWF, and FIU Dissertation Year Fellowship,
all to Laura May-Collado. This research was in part supported by NSF grant DEB-0516038.

Connor RC: Group living in whales and dolphins. In Cetacean Societies: Field studies of dolphins and whales. Edited by Mann J, Connor RC, Tyack PL, Whitehead H. Chicago: The University of Chicago Press; 2000:199-218.